This invention relates to apparatus for receiving and detecting the presence of analog signals on a plurality of input lines. More particularly, it is concerned with receivers for detecting signalling tones on telephone lines.
In a telephone exchange incoming lines may be connected to a receiver which receives signalling tones on the lines, detects the particular tones present, and supplies the tone information in a digital format to control circuitry. Typically, the signalling tones may be of either the well-known MF (multifrequency) or DTMF (dual tone multi-frequency) format. In a system employing the DTMF format the incoming signals are applied to a pair of band reject filters which separate high frequency tones from low frequency tones. The outputs of the band reject filters are applied to comparator and limiter circuitry. The resulting signals are applied to banks of tone filters each of which passes one of the frequency tones used in the system. The output of each tone filter is connected to a level detector which is set by a signal passing through its associated tone filter to produce a signal indicating that a particular tone is present on the incoming line.
In an exchange as described briefly hereinabove, a complete receiver is required for each incoming line on which signalling tones are being received. Thus, the space requirement, weight, and cost increases proportionally with the number of receivers in the exchange.